1. Technical Field
The present invention relates generally to valves, and more particularly, to valve alignment to maintain a seal.
2. Background Art and Technical Problems
Air data systems, which respond to air pressure to determine various parameters such as altitude, airspeed, and the like, are common in most modem aircraft, especially large aircraft. Before air data systems are actually implemented, however, the systems are typically ground tested for operability and accuracy. Air data testers (ADTS) have become important equipment for such testing. An ADT is used to simulate the pneumatic pressures encountered at various speeds and altitudes. Typically, the ADTs are used for testing aircraft controls and calibrating instruments. For safety and efficiency, these controls and displays tend to be very accurate. Accordingly, to obtain this accuracy, the ADTs must also be highly precise, often accurate within 1 percent of the rate of change in altitude or less. Furthermore, the ADTs are preferably able to change the output pressure quickly to simulate rapid altitude changes. Examples of typical pneumatic testers are disclosed in U.S. Pat. No. 4,131,130 entitled "Pneumatic Pressure Control Valve" and issued Dec. 26, 1978 to Joseph H. Ruby and are generally described below.
FIG. 1 shows a typical configuration for existing ADT pressure control valves, examples of which are the Honeywell ADT-222B, -222C and -222D Air Data Test Systems. These ADTs are comprised of a two-input system, whereby one input supplies a positive pressure and another input supplies a negative pressure (a vacuum) which act in conjunction to produce a desired output pressure. The position of a flapper valve structure between the two input ports controls the amount of gas supplied to or withdrawn from a load volume to maintain the desired pressure.
Early designs included a single flapper alternating between covering the two ports. The single flapper design, however, results in wasted air flow as the flapper swings back and forth between the ports. A more modem flapper structure uses a dual flapper, one to cover each of the input ports. The dual flapper decreases wasted air flow in comparison to single flapper designs.
Dual flappers typically employ small gaps between the flappers and the input ports, which further decrease wasted air flow. In particular, ADTs with dual flapper pressure control valves often have gaps between the flapper structure and the input port in the range of 0.0006 inches on the exhaust (vacuum) input side, to 0.0010 inches on the pressure input side of the pressure control valve 100.
To achieve the desired pressure rapidly with such small gaps, dual flappers are commonly designed to elastically deform slightly when pressed against the respective ports. The deformation allows the gap between the opening pressure input to continue widening, while the closed pressure input remains closed, thus enabling faster pressure changes.
Deformation of the flapper, however, may result in an imperfect seal between the flapper and the port. Referring now to FIG. 2, the ideal contact between the flapper 160 and input port 120 allows no air flow, whereas the other port (not shown) remains open to facilitate air flow. In conventional dual flapper ADT systems, however, perfect seal-off occurs only at one particular point of operation, i.e., when the flappers 160 and input ports 120 are in perfect alignment. Thus, at any other operation point, inadvertent air flow may occur through both input ports 160, resulting in wasted air, imprecise output pressure, and the slower pressure changes.
Additionally, to obtain even one point where perfect seal-off is achieved, the assembly of the pressure control valve demands extreme precision. If the flapper structure is not perfectly aligned, perfect seal-off is rarely or never achieved, disrupting the operation of the valve. To properly align the flapper, an experienced craftsman manually repetitiously adjusts and calibrates each feature of the flapper structure. Such features adjusted include, among others, the gaps, lengths, and angles of the flapper structure relative to the ports.
When actually calibrating the dual flapper pressure control valve, the craftsman first adjusts one feature of the pressure control valve, for example, the gap between the flapper and nozzle. He then tests the valve, readjusting the gap as necessary. This process is repeated several times, until the craftsman obtains the proper calibration. The craftsman then adjusts another feature, such as the angle of the flapper, and tests the valve again. However, this time, not only must the craftsman go through the adjust and test process for the angle of the flapper, he must also continually readjust the gaps, as the gaps change with adjustment of the flapper angle. The entire process is repeated many times for each feature adjusted until the entire valve structure is properly aligned. This calibration process can take anywhere from 8 to 10 hours for an experienced craftsman, to as high as 30 hours for less experienced craftsmen.
In addition, even if the one point of perfect seal-off is achieved, any position other than the perfect seal point disrupts the seal between the flapper and the nozzle. For example, referring now to FIG. 3, when the flapper makes first contact with the nozzle, a gap exists at the top of the nozzle. This is due to the angle of flapper as it moves through its range of motion. Until enough force is exerted by the torque motor to cause the flapper to begin deforming and contact the entire nozzle, perfect seal-off does not occur. Meanwhile, as the flapper deforms to seal the nozzle, the gap between the other flapper and pressure input continues to widen, thus wasting air flow, detracting from the precision of the system, and slowing the rate of pressure changes.
Further, as shown in FIG. 4, as the control system drives the flapper structure to continue widening the gap between the flapper and one nozzle, the increasing force exerted on the opposite flapper may cause the opposite flapper to deform past the point of perfect seal-off, forming a gap at the bottom of the nozzle. This gap widens as the force exerted by the torque motor increases. Again, perfect seal-off is lost.
Further, imprecision in the control system, torque motor, and flapper structure may contribute to imperfect seal-off. For example, if the control system directs too much current to the torque motor (e.g. an overdrive situation), the flapper may deform excessively and reduce the effectiveness of the seal, as shown in FIG. 3. Likewise, if the control system directs too little current to the torque motor, the flapper may not deform enough to form a full seal, as shown in FIG. 4. Improper calibration of many other components of the pressure control system may similarly affect the quality of the seal.